electrolessa,Universityform23resistanceAppliedSurfaceScience252(2006)1.IntroductionMagnesiumisthelightestofallmetalsusedasthebaseforconstructionalalloys.Therequirementtoreducetheweightofcarcomponentshastriggeredrenewedinterestinmagnesium.Inaddition,theuseofmagnesiumalloysinotherfields,suchasaerospace,electronicandtelecommunicationcomponents,hasalsoincreasedsteadilyinrecentyears.Thegrowthrateoverthenext10yearshasbeenestimatedtobe7%perannum.However,magnesiumisintrinsicallyhighlyreactiveanditsalloysusuallyhaverelativelypoorcorrosionresistance,whichisactuallyoneofthemainobstaclestotheapplicationofmagnesiumalloys13.Electrolessnickelphosphoruscoatingonmagne-siumalloysisgenerallybetterthantheelectroplateddepositsbecauseoftheirenhancedcorrosionresis-tanceandtheiruniformityachievableoncomplexobjects.Electrolessnickelphosphoruscoatingonmagnesiumalloysisoneofthemostappropriatemethodstofurtherenhancethecorrosionresistance.*Correspondingauthor.Tel.:+862483687731；fax:+862423981731.E-mailaddress:(J.Li).0169-4332/$seefrontmatter#2005ElsevierB.V.Allrightsreserved.doi:10.1016/j.apsusc.2005.04.028Keywords:Magnesiumalloy；Electrolessnickelplating；Porosity；CorrosionAbstractInthepresentpaper,theporosityoftheplatingcoatingwasevaluatedbythecombinationofcorrodkoteandfilterpaper,theeffectsoftheplatingsolutionontheporositywereinvestigated,andthepropertiesoftheporouscoatingswerestudiedthroughscanningelectronmicroscopy(SEM)andelectrochemicalpotentiodynamicpolarization.TheresultsshowthattheeriothromeblackTindicatorusedasanindicatorofthecoatingporosityforcoatingsonmagnesiumalloyismoreeffectivethanmagnesonindicatorandsodiumalizarinesulfonateindicator.Theporosityinelectrolessnickeldepositsonmagnesiumalloywaswellevaluatedbythecombinationofcorrodkoteandfilterpaper.Itisrevealedthattheporesexistonbothgrainsurfaceandgrainboundaries.Anaffectingtrendoftheplatingbathparametersonthecoatingporositywasobtained.#2005ElsevierB.V.Allrightsreserved.StudiesoftheporosityinonmagnesiumJianzhongLia,*,YanwenTianaSchoolofMaterialsandMetallurgy,NortheastbDepartmentofChemistry,SimonFraserReceived2December2004；receivedinrevisedAvailableonlinenickeldepositsalloyZhenqiHuanga,XinZhangbernUniversity,Shenyang110004,China,Burnaby,BC,CanadaV5A1S623April2005；accepted23April2005M/locate/apsusc28392846Eachsystemhasitsadvantagesanddisadvantages.Intheelectrolessplatingprocess,alargeamountofH2gasbubblesandimpurityparticlesareproduced,andthatmayleadtotheporeformation48.However,thenickelmagnesiumsystemisaclassicalexampleofcathodiccoatingonananodicsubstrate.Hence,coatingporositymightinfluencethecorrosionbehaviorandservicelifetimeoftheelectrolessnickel-platedmagnesium.Theprotectiveabilityofelectrolessnickelonmanyengineeringmaterialsislimitedbythecoatingporosity.Inaddition,thedensityandtoughnessofcoatingisintensivelyinfluencedbythecoatingporosity.Itisofactualsignificancetoinvestigatetheporosityoftheelectrolessplatingcoatingonmagne-corrosionresistanceofNiPcoatingbrokengradually1315.Accordingtoacolorreactionbetweentheelementofthesubstrateandthecorrosionsolutionpassingthroughtheopeningholes,thecoatingporositywasevaluatedbythecombinationofcorrodkoteandfilterpaperinthisresearch.Theeffectoftheplatingparametersonthecoatingporositywasstudiedindetail.2.Experimental2.1.ElectrolessplatingonmagnesiumalloyThesubstratematerialusedintheresearchwasAZ91Dcastingmagnesiumalloy.ThechemicalcompositionofthealloyisgiveninTable1.Thesampleswithasizeof50mmC240mmC220mmwereusedintheexperiment.Thesubstratesweremechanicallypolishedwithemerypapersupto1000grittoensuresimilarsurfaceroughness.ThepolishedsampleswerethoroughlywashedwithJ.Lietal./AppliedSurfaceScience252(2006)283928462840#Distilledwaterrinsingsiumalloysindetail.Sofar,therearefewreportsonhowtoevaluatetheporosityoftheelectrolessplatingcoatingonmagnesiumalloysandwhateffectofthebathparametersonthecoatingporosity915.ThestructureofthecoatingporositycanbeexpressedasshowninFig.1.Thesearetwokindsofpore:oneistheopeningholes(D,E),theothersistheclosedone(A,B,C).Theclosedholesmostlyhaveaneffectontheeffectivethicknessandstressofthecoating,buttheopeningholewillaffectthecorrosionresistanceoftheplatingcoating.TheopeningholeDcanbecausedbythedeficiencyofthesubstrate.Thereasonisthatthedeficientspotofmagnesiumalloyseasilyreactswiththeplatingbath,producingH2gas.Thiswillleadtothediscontinuouselectrolessplatingonthedeficientspotofmagnesiumalloys.ContinuousdepositionofNiPcoatingresultsintheoverlapoftheporesothattoformtheopeningholeE.Aprimarycellmaybeformedbetweenthecorrosionsolutionandthesubstratethroughtheopeninghole,leadingtotheFig.1.Shapestructureoftheporosityofthecoating.#Picklingin125gLC01chromicacidand110mLLC01nitricacidfor4560s#Distilledwaterrinsing#FluorideactivationinHF(250mLLC01,70%HF)solutionfor10min#DistilledwaterrinsingTable1ChemicalcompositionoftheAZ91Dalloy(inwt.%)AlMnNiCuZnCaSiKFeMg010.0010.640.010.010.010.001BalanceTable2Thepre-cleaningprocedureUltrasonicdegreasinginacetone#Immersionin10%NaOHaqueoussolutionat608Cfor5minJ.Lietal./AppliedSurfaceScience252(2006)283928462841distilledwaterbeforethepre-cleaningprocedureasshowninTable2.Immediatelyafterthefluorideactivation(thelaststepinthepre-cleaningprocedure),thespecimenswerequicklytransferredtotheplatingbath(1000mL)inaglasscontainerplacedinawaterbathwithaconstanttemperatureof808C.Afreshbathwasusedforeachexperimenttoavoidanychangeinconcentrationofbathspecies.ThebathcompositionsandtheplatingparametersusedintheexperimentsaregiveninTable3.2.2.PorosityevaluationThecoatingporosityofthesampleplatedfor1hwasevaluatedbythecombinationofcorrodkoteandfilterpaper.Inaddition,thecorrespondingcoatingthickness(mm)ofthesampleplatedfor1hwascalculatedfromtheweightgain.ThecorrodkoteTable3ThebathcompositionandplatingparametersBathspeciesandparametersQuantityBasicnickelcarbonate(2NiCO3C13Ni(OH)2C14H2O)(gLC01)11.5Sodiumhypophosphite(gLC01)20Butanediacid(gLC01)5Ammoniumbifluoride(gLC01)15Hydrofluoricacid(mLLC011Sodiumacetate(gLC01)20Sodiumcarbonate(gLC01Thiourea(mgLC01)1.5AmmoniumhydroxideAdjustingpHpHvalue6.3Temperature(8C)80C62solutionwasmadebydissolving5mLHCland6gNaClin100mLdistilledwater.Whenthecoatedspecimenswereimmersedinthecorrodkotesolutionfor5min,thesubstratestartedtobeerodedbythecorrosionsolutionthroughtheopeningholes.Theionsofsubstrateelementwereproduced.Andthen,thespecimenwasimmersedintoanindicatorsolutionfor3min.Theindicatorsolutionmaybeoneofthreefollowingsolutions:(a)0.5gLC01eriothromeblackTindicatorsolution；(b)0.01gLC01magnesonindicatorsolution；(c)0.1gLC01sodiumalizarine-sulfonateindicatorsolution.Acolorreactiontookplacebetweentheionsofsubstrateelementandthecorrespondingindicator.TheimmersedspecimenwasMorphologyofthehighandlowporosityofcoatingswasanalyzedbyascanningelectronmicroscope.Potentiodynamicpolarizationcurvesweremeasuredinaglasscellwiththecoatedmagnesiumalloyspecimen(exposedarea:1cm2)astheworkingelectrode.Thecounterelectrodewasaplatinumchipandasaturatedcalomelelectrode(SCE)wasusedasthereferenceelectrode.Thescanningratewas0.2mV/s.3.Resultsanddiscussion3.1.EffectofindicatorsontheporosityFig.2showsthevariationoftheporositywithdifferentindicatorsasafunctionofthemassconcentrationratioofhypophosphitetobasicnickelcarbonate.ItcanbeseenfromFig.2thattheporositiesdecreasewiththeincreaseoftheratioofhypopho-sphitetobasicnickelcarbonateintherangeof1.01.75,andthentheporositiesincreasewiththeratiointherangeof1.752.5.Thereasonsmaybethattheautocatalyticelectrolessnickeldepositionisinitiatedbycatalyticdehydrogenationofthereducingagentwiththereleaseofhydrideions,inwhichthenthehydrideionssupplyelectronsforthereductionofnickelion,accompaniedtheH2gasproduced.Whentheratioisintherangeof1.01.75,whichmeansaimmediatelydrawnontoapieceoffilterpaper.Theporositywasindicatedbythecolorpointsonthefilterpaper.Theporositywascalculatedaccordingtothefollowingequation(2.1)8,16:Porosityns(2.1)wherenisthenumberofcolorspots；sthesurfaceareaofthecoating.Inaddition,ntakescountofonespot,whenthediameterofcolorspotisbelow1mm；ntakescountofthreespotsasthediameterofcolorspotintherangeof13mm；Intherangeofabove3mm,ntakescountof10spots.Triplicateexperi-mentswereconductedineachcase,andthefinallyevaluatedporosityofthecoatingistheaverageofthreeexperimentresultstoreduceoccasionaldataerrors.2.3.CharacterizationsoftheporouscoatingsJ.Lietal./AppliedSurfaceScience252(2006)283928462842relativelylowerconcentrationofthereducingagent,thedrivingforceofthereactionisdecreased,leadingtoaslowergrowthrateoftheplatingcoating.Slowergrowthrateresultsinmoredefectsonthesubstratesurfaceduetotheaciderosionandthushighercoatingporosity.Withtheincreaseoftheratio,thegrowthrateofcoatingincreases.ThisisalsoevidencedbythecorrespondingcoatingthicknesslabeledinFig.2.TheincreaseofgrowthratehelpstopreventthesubstrateFig.2.Porositydataoftheplatingcoatingsresultedfromdifferentindicators.surfacefromacidcorrosionandthusadecreaseofcoatingporosity.Atlowerratio,thecoatingporosityismainlyresultedfromthedefectsonthesubstratesurface.Intherangeoftheratioabove1.75,muchlargeramountofH2gasisproduced,andtheplatingbathmaydecomposeorprecipitatespontaneouslyduringplating,resultinginasmallereffectiveconcentrationofthereducingagent.Thegrowthratedecreaseswiththeratio,duetothelargeamountofgasbubblesorprecipitatestrappedinthecoatings.Therefore,higherratioalsoresultsinhigherporosity7,8.Thisindicatesthatathigherratiotheimpurityparticlesandgasbubblesintheplatingbathhavearemarkableeffectonthecoatingporosity.Itshouldbealsonotedthatcoatingswithhigherporosityofferlessgrowthsitesoncoatingsurface,resultinginalowergrowthrateduringplating.ThisisevidencedbyFig.2,inwhichanincreaseofcoatingporosityisaccompaniedbyadecreaseofcoatingthicknessandviceversa.ItcanbeseenfromFig.2that,forthesamplespreparedundersameconditions,theporosityvaluesfromtheeriothromeblackTindicatorarebiggerthanthosefrommagneson,andthecorrespondingporosityvaluesfromsodiumalizarinesulfonateindicatorarethelowest.Theoretically,theporosityofthecoatingspreparedundersameconditionsshouldbethesame.Basedonobservations,thedifferenceinmeasuredvaluesofcoatingporositycanbeexplainedasthefollowing.Whenthecoatingporosityisevaluatedwiththemagnesonindicator,magnesiumionsfromthesubstratereactwiththemagnesonindicatortoformaninsolublemagnesiumcomplexleadingtothecorre-spondingcolorreactionintheopeningholes.Theinsolublemagnesiumcomplexmakesithardertorevealtheholeswithsmallerdiametersonfilterpaper.Whenthecoatingporosityisevaluatedwiththesodiumalizarinesulfonateindicator,aluminumionsfromthesubstratereactwiththeindicatortoformaninsolublealumin